Due to their strong adsorption of hydrogen, high catalytic activity and thermal stability, Raney nickel alloy catalysts are widely used in many industrial processes and organic synthesis reactions, such as hydrogenation reactions of the unsaturated compounds olefins, alkynes, nitriles, diolefins, aromatics, carbonyl-containing substances, and even macromolecules with unsaturated bonds, as well as hydrogenation reactions of soluble sugars, such as the hydrogenation of soluble sugars to produce sorbitol and xylitol. Acid is produced in the course of some reactions; under acidic conditions, nickel releases hydrogen and thereby produces nickel ions Ni2+, with the result that the catalyst slowly dissolves and loses its hydrogenating activity. In general, an alkali must be added to the reaction system to neutralize acid, in order to maintain the stability of the nickel catalyst. The addition of alkali will not only increase the cost of the alkali starting material, but also increase the cost of product separation and purification, and will even change the selectivity of the catalyst for the target product. For example, in a reaction in which ethylene glycol is prepared by direct hydrocracking of sugar, since sugar very readily undergoes a hydrolysis side reaction under high-temperature aqueous phase conditions, small-molecule substances such as acetic acid, lactic acid and formic acid are produced, causing an increase in the acidity of the system (Sevilla M, Fuertes A B. Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides. Chemistry-A European Journal. 2009, 15(16): 4195-4203.); it is reported in the literature that the stability of the nickel-containing catalyst can be maintained by regulating the pH of the reaction system at 7 or above (CN 103667365 A). However, under high pH conditions, the yield of propylene glycol will significantly increase while the yield of ethylene glycol will significantly decrease (U.S. Pat. No. 5,107,018, CN 101781167 A, CN 101781171 A, CN 101781166 A); at the same time, acids produced in the hydrolysis side reaction such as formic acid, acetic acid and lactic acid increase, and the total diol yield will correspondingly fall (CN 101544537 A).
Under acidic conditions of pH<5, reducing sugars are in a more stable state, and essentially do not undergo a hydrolysis side reaction (Li Yan, Shen Canqiu et al., Research on the decomposition mechanism of sucrose in impure sugar solutions, China Beet and Sugar, 1996(2): 11-16); thus, the polyol yield of a sugar hydrogenation catalytic system can be increased if the latter operates under acidic conditions. However, under low pH conditions, only precious metals such as Ru and Pt are stable, so can be used as catalytically active components. The use of precious metals will significantly increase the diol production cost. To reduce the amount of precious metal used and increase catalytic activity, supports with a high specific surface area are generally selected to fix and disperse it. However, commonly used supports, for example inorganic oxides such as alumina, silica and magnesia, are unstable under acidic conditions, and readily undergo a neutralization reaction and dissolve in the reaction system, leading to a fall in the polyol yield (CN 103159587 A). Being an acid-resistant material, activated carbon is also commonly used as a catalyst support, to increase the specific surface area of the catalyst (CN 103420796 A, CN 102643165 A, CN 102731258 A, CN 10161325 A). However, activated carbon is also unstable under high-temperature hydrogen conditions, and readily undergoes a hydrogenation reaction in which it is methanized (US 2002/0169344).
Furthermore, nickel alloy materials also include Hastelloys, the principal composition of which is Ni 50-64%, Mo 15-30% and Cr 14-21%. It has an extraordinary capacity for resisting various industrial chemistry environments, and in particular is capable of resisting corrosion by various organic acids; the high molybdenum and chromium contents increase the corrosion resistance thereof. As a corrosion-resistant metal structural material, it lays more emphasis on ensuring mechanical properties.
Thus, there is a need to develop an acid-resistant, cheap and stable nickel alloy catalyst which has no need for a support, can be stably used in continuous industrial production, and can lower the cost of production.